Method and apparatus for side wall passivation for organic etch

ABSTRACT

A robust method for etching an organic low-k insulating layer on a semiconductor device, as disclosed herein, includes introducing into a processing chamber a substrate with an organic insulating layer and an overlying mask layer having an aperture. A plasma is then developed within the chamber from an oxidizing gas and a passivation gas. The passivation gas is preferably either a silicon containing gas or a boron containing gas, or both. The ratio of the oxidizing gas to the passivation gas is preferably at least 10:1. In addition, an inert carrier gas may be provided. The plasma is then used to etch the organic insulating layer through the mask layer, thereby forming a via having essentially vertical sidewalls in the organic low-k insulating layer.

BACKGROUND OF THE INVENTION

The present invention relates generally to semiconductor processing and,more particularly to methods for etching organic insulating layers.

The present day semiconductor industry continually strives to increasedevice performance by reducing device dimensions and increasing devicepacking densities. For a given chip size, increasing the device packingdensity can be achieved by reducing the vertical and lateral distanceseparating active devices, with a resulting reduction in dielectricthickness (often referred to as inter-metal oxide or IMO) betweenlayers. Unfortunately, reducing dielectric thickness increasesinterlayer capacitance, which results in diminished high frequencyperformance of the integrated circuit.

In integrated circuits, conventional insulating layers, such as silicondioxide and silicon nitride, generally have dielectric constants “k” ofabout 3.9 and above. For example, the dielectric constant of silicondioxide is about 3.9 and the dielectric constant of silicon nitride isabout 9.0. As the feature scales shrink in IC devices it becomesdesirable to reduce the dielectric constant of the insulating layer toreduce the inter-layer capacitance.

Since the dielectric constant “k” of some organic materials, such asFLARE from AlliedSignal, Inc. and SiLK from Dow Chemical, is generallyless than 2.7, organic materials can be used as a low-k organicinsulating layers for chip fabrication. However, the organic materialspresent problems during a conventional etch process. Most notably,etching organic materials using conventional methods results in theorganic insulating layer having via sidewalls that are undercut andbowed.

FIG. 1A is an illustration showing a cross-sectional view of a prior artintegrated circuit structure 10 having an organic insulating layer 12prior to a plasma etch. More particularly, integrated circuit structure10 includes an organic insulating layer 12, a silicon dioxide hardmasklayer 14 disposed above the organic insulating layer 12, and an organicresist mask 16 formed above the silicon dioxide hardmask layer 14. FIG.1B shows the integrated circuit structure 10 after etching the silicondioxide hardmask layer 14 by a suitable oxide-etch process though theresist mask 16. FIG. 1C shows the integrated circuit structure 10 afteretching (“oxidizing”) the organic insulating layer 12 using conventionaloxygen containing gases. A conventional etch of organic low-k materialsin a plasma chamber typically employs oxygen containing gases such asO₂, CO₂, and SO₂. In the presence of plasma, oxygen atoms and ions areformed. Atomic oxygen reacts with organic material and forms CO, H₂, andCO₂ as by products. However, spontaneous reactions between the atomicoxygen and the organic low-k materials occur. The reactions between theatomic oxygen and the organic low-k insulating layer cause isotropicetching, which results in undercut and bowing of the organic insulatinglayer 12 as illustrated by the bowed profile of sidewalls 18.

To counter undercut and bowing in the sidewalls 18, gases such as C₂H₄are sometimes used during organic etch. FIG. 1D shows the integratedcircuit structure after etching the organic insulating layer 12 usingC₂H₄ containing gases. The C₂H₄ forms a C_(x)H_(y) polymer on the etchsidewall during the etch process. The result is a sidewall 18 which issometimes less undercut and bowed than the sidewall profile resultingfrom conventional oxygen containing gases. However, this approach isdifficult to control, and does not always result in an improved sidewall18 profile. The amount of improvement depends on the parameters used tocontrol the sidewall 18 profile, resulting in an approach which isdelicate and lacking robustness.

Another approach used to counter undercut and bowing in the sidewall 18profile during an organic etch is the use of high energy sputtering.Prior art FIG. 1E shows the integrated circuit structure 10 afteretching the organic insulating layer 12 using high energy sputtering.The high energy sputtering causes sputtering of SiO₂ from the silicondioxide hardmask layer 14 to create sidewall passivation. However, thehigh energy sputtering can cause damage 20 to the hardmask layer 14during the etch process. In addition, when a deep etch is needed, highenergy sputtering is often insufficient to cause sidewall passivationcoating near the bottom of the via, again resulting in an undercut andbowed sidewall 18 profile.

All of the organic etch approaches discussed above fail to provide aconsistent, robust etch process which provides a sidewall profile in anorganic insulating layer that is not undercut or bowed. Accordingly,there exist a need for a robust organic etch process that does not causedamage to the IC, and provides better sidewall profiles.

SUMMARY OF THE INVENTION

The present invention meets the aforementioned requirements by providinga process that etches organic insulating layers utilizing an oxidizinggas and a passivation gas. The passivation gas reacts with oxygen atomsor oxygen molecules to form a nonvolatile passivation film whichdeposits on the sidewalls of vias being formed in the organic insulatinglayer. The passivation film provides sidewall passivation whichessentially inhibits isotropic etch of the organic insulating layer.Thus, the resultant via sidewall profile in the organic insulating layeris essentially vertical with respect to the plane of the insulatinglayer.

One aspect of the present invention teaches a method for anisotropicallyetching an organic insulating layer through an aperture in a mask layer.A substrate, with an organic insulating layer and an overlying masklayer having an aperture, is introduced into a processing chamber. Aplasma is then developed within the chamber from an oxidizing gas and apassivation gas. The passivation gas is preferably either a siliconcontaining gas or a boron containing gas, or both. The ratio of theoxidizing gas to the passivation gas is preferably at least 10:1. Aninert carrier gas may also be provided. The plasma is then used to etchthe organic insulating layer through the mask layer.

Another aspect of the present invention teaches an etch system fororganic layers. The organic etch system includes a chamber which isreceptive to a substrate provided with an organic insulating layer to beetched. Also included is a gas inlet mechanism connecting an oxidizinggas and a passivation gas source. The passivation gas is derived fromthe group including silicon containing gases and boron containing gases.The ratio of the oxidizing gas to the passivation gas is preferably atleast 10:1. Further included in the system is a pair of electrodesdisposed within the chamber, and an RF generator coupled to theelectrode pair so that a plasma is formed with the oxidizing gas and thepassivation gas which etches exposed portions of the organic insulatinglayer.

As stated above, the present invention has the ability to produceaccurate vias having essentially vertical sidewall profiles in organicinsulating layers. The ability to produce accurate vias allows the useof organic low-k insulating layers in integrated circuit fabrication.The organic low-k insulating layers lower the interlayer capacitance,and thereby increase high frequency performance of the integratedcircuit.

These and other advantages of the present invention will become apparentto those skilled in the art upon a reading of the following descriptionsand a study of the various figures of the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention, together with further advantages thereof, may best beunderstood by reference to the following description taken inconjunction with the accompanying drawings in which:

FIG. 1A is an illustration showing a cross-sectional view of a prior artintegrated circuit structure prior to a plasma etch;

FIG. 1B is an illustration showing a cross-sectional view of the priorart integrated circuit structure after etching the silicon dioxidelayer;

FIG. 1C is an illustration showing a cross-sectional view of the priorart integrated circuit structure after etching the organic insulatinglayer using conventional oxygen containing gases;

FIG. 1D is an illustration showing a cross-sectional view of the priorart integrated circuit structure after etching the organic insulatinglayer using C₂H₄ containing gases;

FIG. 1E is an illustration showing a cross-sectional view of the priorart integrated circuit structure after etching the organic insulatinglayer using high energy sputtering;

FIG. 2A is an illustration showing a cross-sectional view of anintegrated circuit structure prior to a plasma etch in accordance with apreferred embodiment of the present invention;

FIG. 2B is an illustration showing a cross-sectional view of anintegrated circuit structure after etching the silicon dioxide layer inaccordance with a preferred embodiment of the present invention;

FIG. 2C is an illustration showing a cross-sectional view of anintegrated circuit structure after etching the organic insulating layerusing a silicon containing passivation gas in accordance with apreferred embodiment of the present invention;

FIG. 3 is a flow chart showing a method for etching an organicinsulating layer in accordance with one embodiment of the presentinvention;

FIG. 4 is an illustration showing an organic insulating layer etchingsystem in accordance with one embodiment of the present invention; and

FIG. 5 is an illustration showing an organic insulating layer etchingsystem having multiple gas inlets in accordance with one embodiment ofthe present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIGS. 1A-1E were described in terms of the prior art. A preferredembodiment of the present invention will now be described with referenceto FIGS. 2A-2C. FIG. 2A is an illustration showing an integrated circuitstructure 30 prior to plasma etch in accordance with a preferredembodiment of the present invention. The integrated circuit 30 includesan organic low-k insulating layer 32, a silicon dioxide layer 34disposed above the organic low-k insulating layer 32, and an organicresist mask 36 formed above the silicon dioxide layer 34.

A process in accordance with the present invention begins with an etchof the silicon dioxide layer 34 preferably utilizing a gas such as CF₄.Other gases such as C₂F₆, CHF₃, and SF₆ are also suitable for use in theetching process. Process parameters for etching silicon dioxide are wellknown to those skilled in the art. The result of the etch is shown inFIG. 2B.

After etching the silicon dioxide layer 34, a mixture of a passivationgas and an oxidizing gas are formed into a plasma to anisotropicallyetch the organic insulating layer 32 through an aperture 35 in thesilicon dioxide layer 34. FIG. 2C is an illustration showing theintegrated circuit structure 30 after etching the organic low-kinsulating layer 32 using a passivation gas added to an oxygencontaining plasma. The passivation gas can be either a siliconcontaining gas, such as SiH₄, SiF₄, or SiCl₄, or a boron containing gas,such as BCl₃.

More particularly, plasma including a passivation gas and an oxidizinggas is used to etch vias in the organic low-k insulating layer 32. Thepassivation gas reacts with oxygen atoms or oxygen molecules to form anonvolatile passivation film 38 during this process. The type ofpassivation film 38 is determined by the type of passivation gasutilized, for example, silicon containing gases result in an SiO₂passivation film, while boron containing gases result in a B₂O₂passivation film. The passivation film 38 deposits on the sidewalls ofthe vias 39 as they form in the organic low-k insulating layer 32.Normally, spontaneous reactions between atomic oxygen from the plasmaand the organic insulating layer cause isotropic etching, which resultsin undercut and bowing in the sidewall 40 profile. However, the sidewallpassivation film 38 of the present invention provides sidewallpassivation which essentially inhibits isotropic etch resulting fromspontaneous reactions between atomic oxygen and the organic insulatinglayer. Thus, the sidewall 40 profile of the present invention isessentially vertical with respect to the plane of the insulating layer.

Referring next to FIG. 3, a method 100 for etching an organic low-kinsulating layer in accordance with one embodiment of the presentinvention will now be described. In an initial operation 102, anintegrated circuit is prepared for the organic insulating layer etchprocess. Typically, this preparation includes etching the silicondioxide layer with a fluorocarbon-containing gas such as CF₄, C₂F₆,CHF₃, and/or SF₆. As will be apparent to those skilled in the art,fluorine from the fluorocarbon-containing gas reacts with the silicon inthe silicon dioxide layer during the etch process. The actual processparameters for etching silicon dioxide are also well know to thoseskilled in the art. Having etched the silicon dioxide layer, theintegrated circuit structure is then ready for the organic insulatinglayer etch.

The method 100 continues with an organic insulating layer etch, in anoperation 104. After etching the silicon dioxide layer, a mixture of apassivation gas and an oxidizing gas are formed into a plasma toanisotropically etch the organic insulating layer through an aperture inthe silicon dioxide layer. The passivation gas can be either a siliconcontaining gas, such as SiH₄, SiF₄, or SiCl₄, or a boron containing gas,such as BCl₃. In addition, the oxygen to passivation gas ratio in theplasma preferably does not exceed 10:1. However, during an organicinsulating layer etch having a longer etch time, the oxygen topassivation gas ratio is typically about 100: 1, in order to avoid overpassivation of the via sidewalls. Over passivation of via sidewalls maycause etch stop and sloped via sidewalls from passivation film buildup,and therefore should be avoided. On the other hand, during a highlyactive organic insulating layer etch having a shorter etch time, theoxygen to passivation gas ratio is typically about than 50:1, in orderto avoid under passivation of the via sidewalls.

Using these ratios of oxygen containing gas to silicon containing gas,the organic etch rate is in the range of about 0.4-2.0 μ/min, resultingin an organic layer etch time of between 1-2 minutes.

To elaborate further, the plasma, including the passivation gas and theoxidizing gas, is used to etch vias in the organic low-k insulatinglayer. The passivation gas reacts with oxygen atoms or oxygen moleculesto form a nonvolatile passivation film. The type of passivation filmformed is determined by the type of passivation gas utilized in theplasma. For example, silicon containing gases result in an SiO₂passivation film, while boron containing gases result in a B₂O₂passivation film. The passivation film deposits on the sidewalls of thevias as they form in the organic insulating layer. Normally, spontaneousreactions between atomic oxygen from the plasma and the organicinsulating layer cause isotropic etching which results in undercut andbowing in the sidewall profile. However, the sidewall passivation filmof the present invention provides sidewall passivation which essentiallyinhibits isotropic etch resulting from spontaneous reactions betweenatomic oxygen and the organic low-k insulating layer. Thus, the sidewallprofile of the present invention is essentially vertical with respect tothe plane of the insulating layer.

Finally, in an operation 106, the organic low-k insulating layer etchprocess is stopped when the etch reaches an end point. Dry etchequipment used in a typical semiconductor production environmentrequires the availability of effective diagnostic and etch end pointdetection tools. Four common methods for determining the end point ofdry etch processes are: laser reflectivity; optical emissionspectroscopy; direct observation of the etched surface through a viewingport on the chamber, by a human operator; and mass spectroscopy.

Plasma etching systems as described above consist of several components.FIG. 4 is an illustration showing an organic insulating layer etchingsystem 50 in accordance with one embodiment of the present invention.The organic insulating layer etching system 50 includes a chamber 52receptive to a substrate 62 provided with an organic insulating layer tobe etched, a gas inlet mechanism 54 connecting to an oxidizing gas and apassivation gas source 56, a pair of electrodes 58 disposed within thechamber 50, and an RF generator 60 coupled to the electrodes 58. In someinstances the upper electrode can by omitted by grounding the RFgenerator 60 to the chamber 52.

After the substrate 62 is prepared for the organic low-k insulatinglayer etch, it is placed in the chamber 52. The gas inlet mechanism 54is then used to release, into the chamber 52, the oxidizing gas and thepassivation gas from the gas source 56. The RF generator 60 is then usedto create a plasma 61 containing the oxidizing and passivation gases inthe chamber 52. The passivation gas reacts with oxygen atoms, molecules,or ions to form a nonvolatile passivation film on via sidewalls. Thetype of passivation film formed is determined by the type of passivationgas utilized. For example, silicon containing gases result in a SiO₂passivation film, while boron containing gases result in a B₂O₂passivation film. The passivation film deposits on the via sidewallsformed in the organic insulating layer of the substrate 62. Normally,spontaneous reactions between atomic oxygen from the plasma and theorganic low-k insulating layer cause isotropic etching which results inundercut and bowing in the sidewall profile. However, the sidewallpassivation film of the present invention provides sidewall passivationwhich essentially inhibits isotropic etch resulting from spontaneousreactions between atomic oxygen and the organic insulating layer. Thus,the sidewall profile of the present invention is essentially verticalwith respect to the plane of the organic insulating layer.

In another embodiment of the present invention, the oxidizing andpassivation gases are kept separate until released into the plasmacontaining chamber 52. FIG. 5 is an illustration showing an organicinsulating layer etching system 70 having multiple gas inlets inaccordance with one embodiment of the present invention. The organicinsulating layer etching system 70 includes a chamber 52 receptive to asubstrate 62 provided with an organic low-k insulating layer to beetched, multiple gas inlet mechanisms 54 connecting to an oxidizing gassource 56A and a passivation gas source 56B, a pair of electrodes 58disposed within the chamber 50, and an RF generator 60 coupled to theelectrodes 58. As described above, in some cases the upper electrode canbe omitted by grounding the RF generator 60 to the chamber 52.

Some passivation gases, such as SiF₄ and SiCl₄ can be premixed with theoxidizing gas before being released into the plasma containing chamber52. In such cases, the organic insulating layer etching system onlyrequires a single gas inlet 54 and gas source 56, as shown in FIG. 4.However, some passivation gases, such as SiH₄, cannot be premixed withthe oxidizing gas before being released in the plasma containing chamber52, because such passivation gases will react with the oxygen beforebeing released into the plasma. In these cases, multiple gas inlets 54and gas sources 56A, 56B are needed. One gas source 56A contains theoxidizing gas, while the other gas source 56B contains the passivationgas. The gas inlet mechanisms 54 are then used to release the oxidizinggas and the passivation gas into the plasma containing chamber 52 in theproper ratio, usually under automated (e.g., computer) control. Theoxygen to passivation gas ratio in the plasma preferably does not exceed10:1. However, during an organic insulating layer etch having a longeretch time, the oxygen to passivation gas ratio is typically about than100:1, in order to avoid over passivation of the via sidewalls. On theother hand, during a highly active organic insulating layer etch havinga shorter etch time, the oxygen to passivation gas ratio is typicallyabout than 50:1, in order to avoid under passivation of the viasidewalls. Using these ratios of oxygen containing gas to siliconcontaining gas, the organic etch rate is in the range of about 0.4-2.0μ/min, resulting in an organic layer etch time of between 1-2 minutes.

While this invention has been described in terms of several preferredembodiments, there are many alterations, permutations, and equivalentswhich fall within the scope of this invention. It should also be notedthat there are many alternative ways of implementing the methods andapparatuses of the present invention. It is therefore intended that thefollowing appended claims be interpreted as including all suchalterations, permutations, and equivalents as fall within the truespirit and scope of the present invention.

What is claimed is:
 1. A method for anisotropically etching an organicinsulating layer through an aperture in a mask layer, comprising:introducing a substrate provided with an organic low-k insulating layerand an overlying mask layer having an aperture into a processingchamber; and creating a plasma within said chamber having componentsderived from an oxidizing gas and a passivation gas, wherein thepassivation gas is derived from boron containing gasses, wherein theratio of said oxidizing gas to said passivation gas is at least 50:1,whereby said organic insulating layer is etched through said aperture insaid mask layer.
 2. A method as recited in claim 1, wherein the ratio ofsaid oxidizing gas to said passivation gas in at least 100:1.
 3. Amethod as recited in claim 1, wherein the passivation gas is BCl₃.
 4. Amethod as recited in claim 1, wherein the overlying mask includes aplurality of apertures.
 5. A method for making an integrated circuitstructure having an organic dielectric layer provided with a via havingessentially vertical sidewalls, comprising: introducing a substrateprovided with an organic insulating layer and an overlying mask layerhaving an aperture into a processing chamber; and creating a plasmawithin said chamber having components derived from an oxidizing gas anda passivation gas, said passivation gas derived from boron containinggasses, wherein the ratio of said oxidizing gas to said passivation gasis at least 50:1, whereby said organic insulating layer is etchedthrough said aperture in said mask layer.
 6. A method as recited inclaim 5, wherein the ratio of said oxidizing gas to said passivation gasin at least 100:1.